A coil winding wire that is used in a transformer of a switching power supply device assembled in an in-car battery charger or the like needs to be insulated from other electronic components. As a coil bobbin used for the insulation, for example, Japanese Patent Application Laid-Open No. 2010-45187 discloses a coil bobbin of which a main body portion is inserted into a coil winding wire and which has a protrusion portion protruding outward from the outer peripheral surface of the main body portion.